Preheat oven to 400°, take puff pastry out of freezer. Put pan in oven to preheat.
Cut apples into 1-2 cm square pieces
Melt butter in pan and add apples, sugar and spices.
Cook on medium until the apples have softened and released some of their juices. Increase the heat and boil the juices for 2-3 minutes.
Dissolve the starch in about a tablespoon of cold water. Add to the pan and mix throughly until thickened. Remove from the heat and allow to cool.
Roll out puff pastry into large rectangles. Egg wash the edges and put a blog of cooked apples in the middle. Fold over and fork the edges. Egg wash the top and sprinkle with demerara sugar.
Bake for 25-30 minutes. Remove from oven and allow to cool before eating.